Method of and device for echo cancellation

ABSTRACT

Echo cancellation in a full duplex loudspeaking telephone is usually effected by means of an adaptive filter, which by means of a combining (difference) circuit cancels the microphone-signal component picked up by the microphone via the acoustic echo path. For a correct operation of the adaptive filter the levels between the signals at the input line and at the input of the combining circuit should be in a given ratio. For automatically adjusting this ratio an attenuator circuit (26) is arranged before the combining circuit, and at the output of the combining circuit an amplifier (22) is added whose gain is inverse to the attenuation of the attenuator circuit. A control circuit (40) compares the levels of the signals at the input line (10) and at the input of the combining circuit and adjusts the attenuation of the attenuator circuit and the gain of the amplifier accordingly. In order to enable this adjustment to be carried out over a maximal range of acoustic conditions a further attenuator circuit (36) is arranged before the adaptive filter, which further attenuator circuit is also controlled by the control circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of echo cancellation for a full duplexloudspeaking telephone, in which the loudspeaker signal picked up by themicrophone is cancelled by means of an adaptive filter. It also relatesto a device for echo cancellation with an adaptive filter.

2. Description of the Related Art

Such a method and a corresponding device are known, for example from EP0,301,627 B1. Said document describes an adaptive filter which operatessubstantially free from delay. The coefficients of the adaptive filtercan be adjusted only in those time intervals in which there is no speechat this location, so that the microphone then only picks up the echosignal, which is cancelled to an optimum extent by means of the adaptivefilter.

For a correct operation of the adaptive filter it is essential that thelevels of the signals at the input of the adaptive filter and thecombining circuit, i.e. the summing circuit, are in a given ratio to oneanother. This is generally achieved by arranging an amplifier before theloudspeaker and an amplifier after the microphone, the gain of the twoamplifiers being adjusted in such a manner that the ratio between thelevels of the two signals has a given value. However, these levelsdepend on the geometrical position of the loudspeaker relative to themicrophone and on the environment, and the adjustment must be repeatedupon every change of place.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an echo cancelation methodand device for a full duplex loudspeaking telephone, which providessubstantially automatic adjustment of the ratio between the levels ofthe signal at the input of the adaptive filter and the signal generatedby the microphone for the purpose of combining.

Owing to the adjustable attenuation of the microphone signal prior tocombining it is achieved that the desired signal level ratio is adjustedindependently of the loudspeaker amplifier and the microphone amplifierand of the position of the loudspeaker relative to the microphone, thesignal produced at the transmitting line by the microphone beingindependent of this adjustment because the attenuation of the microphonesignal before the transmitting line is cancelled.

The invention is also very advantageous in the case that the gain of theamplifier preceding the loudspeaker is changed, for example, for abetter intelligibility of the other station of the connection, becausethe additional attenuation can then automatically be changed accordinglyin an opposite sense. Since this change in attenuation is also cancelledthe signal at the transmitting line yet remains constant as far as themicrophone signal is concerned. In principle, this also applies to achange of the gain of the microphone amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the sole invention will be described in more detail, byway of example, with reference to the sole drawing. The drawing shows afull duplex loudspeaking telephone having an input line 10 connected toan amplifier 12 whose output is connected to a loudspeaker 14.

The acoustic signal emitted by the loudspeaker reaches a microphone 30via the acoustic echo path 18, which microphone also picks up acousticspeech signals and noises produced at this location. The signalgenerated by the microphone 30 is applied to the transmitting line 20via an amplifier 28, a first attenuator circuit 26, a combining circuit24 and an amplifier circuit 22.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The output signal of the combining circuit 24 appearing on the line 23is also applied to an adaptive filter 34, which receives the signalappearing on the input line 10 at an input 33 via an attenuator circuit36 whose function will be described later. At its output 35 the adaptivefilter 34 generates a filtered signal which, in the case that themicrophone 30 only picks up the acoustic signal via the echo path 18, isopposite to the signal on the line 25, so that the output signal of thecombining circuit 24 appearing on the line 23 is basically zero. As aresult of this, the microphone signal component emanating from the echopath is always cancelled exactly even if the microphone 30 picks upadditional speech signals at this location.

For a correct operation of the adaptive filter the level of the signalon the line 25 should be 6 dB below the level of the signal on the inputline 10. In the present device this is achieved as follows.

The attenuator circuit 26 and the amplifier circuit 22 each have acontrol input connected to a control circuit 40 via a line 41. Theattenuation of the attenuator circuit 26 and the gain of the amplifiercircuit 22 are controlled via the line 41, in such a manner that thegain is inverse to the attenuation, so that the effect of the attenuatorcircuit 26 and that of the amplifier circuit 22 cancel one another. Thecontrol circuit 40 receives the signal at the output of the attenuatorcircuit 26 via the line 25 and the signal on the input line 10 and formsthe ratio between the levels of these two signals. At a given instant,for example in response to a signal from a user, the control circuit 40actuates a signal generator 44, for example a noise generator, via aline 45, which generator produces a signal on the input line 10. Thissignal is applied to the control circuit 40 and, via the line 25, thecontrol circuit receives the signal produced via the loudspeaker 14, theecho path 18 and the microphone 30 and in response thereto changes theattenuation of the attenuator circuit 26 via the line 41 until thelevels of the signals at the lines 10 and 25 have the predeterminedratio of 6 dB. This is then independent of the setting of the gain ofthe amplifier 12 for the loudspeaker 14 and of the gain of the amplifier28 for the microphone 30. It is then assumed that during this adjustmentprocess no other noise or speech signals reach the microphone 30.

The amplifier 12 for the loudspeaker 14 generally has a variable gain inorder to allow the reproduction level to be adapted to localrequirements. If after the adjustment the gain of the amplifier 12 ischanged this change is also transferred to the control circuit 40 viathe line 15, which control circuit changes the attenuation of theattenuator circuit 26 in an opposite sense. If the gain of the amplifier12 is, for example, increased by 3 dB the attenuation of the attenuatorcircuit 26 is also increased by 3 dB, so that consequently the ratiobetween the levels of the signals on the lines 10 and 25 remainsconstant again. Since the attenuation of the attenuator circuit 26 isalways compensated by a corresponding gain of the amplifier circuit 22,the ratio between the levels of the signals at the transmitting line 20and the line 27 connected to the output of the microphone amplifier 28will remain constant.

With the described adjustment process it is possible that the attenuatorcircuit 26 is adjusted exactly to 0 dB in order to obtain the abovelevel ratios. If the gain of the amplifier 12 is then reduced theattenuation of the attenuator circuit 26 would also have to be reduced,i.e. it would have to operate as an amplifier. If this is to be avoidedan attenuator circuit 36 may be arranged before the input 33 of theadaptive filter 34 to provide a suitable attenuation of the signal atthe input line 10 to the adaptive filter 34 rather than to reduce theattenuation of the attenuator circuit 26 beyond 0 dB. This attenuationof the attenuator circuit 36 is adjusted by the control circuit 40 viathe line 43 in such a manner that the difference between theattenuations of the two attenuator circuits 36 and 26 has a valuedetermined by the setting during the adjustment process, if necessarycorrected by a change in the gain of the amplifier 12 via the line 15,in which case it is advantageous if for each setting either theattenuator circuit 36 or the attenuator circuit 26 or both have zeroattenuation. For echo cancellation a change of the gain of the amplifier12 is equivalent to a change of the gain of the microphone amplifier 28,so that if this gain is also variable a signal is applied to the controlcircuit 40 via the line indicated by the dashed arrow 47 and alsoproduces a change of the attenuations of the respective attenuatorcircuits 26 and 36.

It is to be noted that several parts of the present device, particularlythe control circuit 40, the attenuator circuits 26 and 36, the amplifiercircuit 22 and the combining circuit 24, and if desired also thevariable amplifiers 12 and 18, can be realised by means of a singlesignal processor.

I claim:
 1. A method of echo cancellation for a full duplex loudspeakingtelephone, in which method an input signal is reproduced acoustically bya loudspeaker and is simultaneously filtered adaptively; an acousticsignal is converted into a microphone signal by a microphone; and themicrophone signal is combined with the adaptively filtered signal so asto form an output signal; characterized in that prior to combining withthe filtered signal the microphone signal is attenuated to an extentsuch that a fixed ratio is maintained between the level of the inputsignal and the level of the attenuated microphone signal produced by theinput signal via a feedback path from the loudspeaker to the microphone;and in that the attenuation of the microphone signal is cancelled byamplification after being combined with the filtered signal.
 2. A methodas claimed in claim 1, characterized in that prior to reproduction bythe loudspeaker the input signal is amplified by an amplifier having anadjustable gain factor, and upon a change in said gain factor by a givenamount the attenuation of the microphone signal is changed by anequivalent amount so as to maintain said fixed ratio of signal levelsunchanged.
 3. A method as claimed in claim 1, characterized in that theinput signal applied to the adaptive filter is adjustably attenuated,and such attenuation and the attenuation of the microphone signal areadjusted so as to maintain said fixed ratio between the level of theinput signal and the level of the attenuated microphone signalunchanged.
 4. An improved echo cancellation circuit for a loudspeakingtelephone which comprises a receiving line and a transmitting line, aloudspeaker coupled to the receiving line, a microphone coupled to afirst input of a combining circuit (24) having an output coupled to thetransmitting line, and an adaptive filter having an input coupled to thereceiving line and an output coupled to a second input of the combiningcircuit; characterized in that said echo cancellation circuit furthercomprises: a first attenuator circuit with variable attenuation arrangedbetween said microphone and said combining circuit (24); an amplifier(22) arranged after the combining circuit and having a variable gainwhich is inversely proportional to the attenuation of said firstattenuator circuit; and a control circuit (40) for comparing the levelof the signal at the input line with the level of the signal appearingat the first input of the combining circuit and for adjusting theattenuation of the first attenuator circuit so as to obtain a givenratio between said two levels during a given time interval.
 5. An echocancellation circuit as claimed in claim 4, further comprising a secondamplifier (12) with variable gain arranged before the loudspeaker, thecontrol circuit being adapted to change the attenuation of the firstattenuator circuit inversely to the change of the gain of the secondamplifier (12) outside the given time interval.
 6. An echo cancellationcircuit as claimed in claim 4, further comprising a second attenuatorcircuit (36) with variable attenuation arranged before the input of theadaptive filter, the control circuit being adapted to adjust theattenuations of the first attenuator circuit (26) and the secondattenuator circuit (36) so as to obtain said given ratio between the twosignal levels.